1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for reading an image of both sides of a document in a single conveyance and correcting a color difference between image signals for the front and back sides.
2. Description of the Related Art
Color copiers scan a document using a scanner, guide an image reflected by the document to a photoelectric conversion element (e.g., charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS)), process a photoelectrically converted image signal, and form an image using an image forming unit. Such an image forming apparatus reads a first side (front side) of a duplex printed document, reverses the document, and then reads a second side (back side) of the document.
Reading apparatuses capable of reading both sides of a document (hereinafter referred to as duplex document reading apparatuses) using a device called a sheet through document feeder (hereinafter referred to as an SDF) have been developed. This type of reading apparatus includes a first reading sensor (unity-magnification sensor) for reading the front side of a document and a second reading sensor for reading the back side of the document. The first reading sensor is a contact sensor and is disposed within an SDF. The second reading sensor is disposed such that the document and a contact glass are arranged between the second reading sensor and the SDF. A color copier that includes such first and second reading sensors and that can read both sides of a document by causing the document to pass through between the first and second reading sensors has been developed.
An example of this type of reading apparatus using an SDF is an image forming apparatus (e.g., copier) that includes a unity-magnification reading sensor disposed within the SDF and used for reading a document, in addition to a scanner, and that can read both sides of the document without reversing the document by using the scanner together.
The first reading sensor uses a contact unity-magnification sensor as a reading unit for the structural reason that it is difficult to have a reduction optical system within the SDF. The second reading sensor includes a scanner (runner) for reading a thick document, such as a book, and a “stiff” document that is difficult to pass through the SDF, such as thick paper, in addition to a “thin” document readable by the SDF. However, a reduction reading sensor is used as a reading unit because it can have larger depth of focus.
It is preferable that image signals obtained by the first and second reading sensors reading the same document, that is, a first image signal from the first reading sensor and a second image signal from the second reading sensor be at substantially the same value. In shipping adjustment, the difference between values read by both sensors (first and second reading sensors) can be minimized. However, reading features of both sensors may change with time. In this case, it is necessary to correct the changed features using a simple process in a user operating environment.
One approach to addressing this problem is an image forming apparatus described in Japanese Patent Laid-Open No. 2003-032504. This image forming apparatus obtains RGB-γ conversion tables outputted from first and second reading units on the basis of a first image signal from the first reading unit reading a reference pattern and a second image signal from the second reading unit reading the same reference pattern, respectively. The image forming apparatus generates the RGB-γ conversion tables corrected such that the difference between image signals output by both reading units for RGB-γ converted read values is small. By use of the generated RGB-γ conversion features of both reading units, the image forming apparatus can reduce the difference between the read values obtained by reading of the same document by the two reading units.
The image forming apparatus described in Japanese Patent Laid-Open No. 2003-032504 uses a density gradation pattern that is previously stored and set as a reference pattern. Accordingly, by use of a gray-scale pattern having a wide range of colors, color matching can be evenly performed on a wide range of colors.
However, when an operator wants customization of colors or local color matching, it is required to generate a gray-scale optimized to a requested color. In this case, it is difficult for the apparatus described in Japanese Patent Laid-Open No. 2003-032504 to generate such an optimal gray-scale pattern because the apparatus uses a preset gray-scale pattern.
In actual use, for example, in the case of materials for a presentation, a wide range of colors are less prone to being used, and rather, local color matching biased to a specific hue of a logotype with high accuracy is desired.